Electromagnetic interference, or EMI, is a common problem that influences the design of many electrical circuits. EMI results from the undesired interaction and coupling of electric and magnetic fields generated by sources within or external to an electrical circuit. Devices susceptible to EMI suffer in performance. Thus such interference should be minimized.
EMI can be particularly severe when a radio circuit is housed within a small, confined enclosure, such as a cellular or other radiotelephone, and thus unavoidably is positioned adjacent other electronic circuits. The proximity of the radio circuit to other circuit types generally increases both the likelihood of the circuit being subjected to EMI and the intensity of EMI received by the circuit. The combination of radio frequency (RF) circuits with high speed digital circuits further increases the likelihood of EMI between the circuits, as the spurious high frequency signal components associated with the high-rise-time transient signals generated by microprocessors and digital signal processors being clocked at MHz rates can easily couple to RF circuits.
EMI may arise from either a defect in circuit or packaging design. Improperly designed circuits, such as those in which the potential coupling of adjacent circuits has not been considered and addressed, may allow spurious signals to "leak in" through power supply lines. Injudicious decisions made during circuit layout which result in poor grounding and poor component placement can also produce EMI.
As noted above, packaging can also greatly influence the degree to which EMI affects the performance of a radio circuit. The underlying goal of RF packaging is to provide adequate shielding of internal circuitry from compartmental RF energy leakage and to prevent external energy leakage. In addition, the packaging design should provide rugged, low impedance electrical grounding.
The design of the shielding apparatus is dependent upon the nature of the interference. High impedance electromagnetic waves, which are primarily electric in nature, can be effectively blocked and dissipated by a well grounded conductor. Low impedance electromagnetic waves, which in contrast are primarily magnetic in nature, can be blocked by shields formed from a material with high magnetic permeability, such as steel or mu-metal. The configuration of many RF circuits requires that the designer consider and address either or both low and high impedance-based EMI.
Shielding devices have been disclosed in a variety of configurations for solving a specific shielding problem at hand. U.S. Pat. No. 4,572,921 issued Feb. 25, 1986, to May et al. relates to a shielding device intended for application on doors and hatches of enclosures of the type often included on an anechoic chamber. The shield comprises an elongate conductive strip having a set of fingers that extend transversely from, then overlie the strip. The shield is received within a door channel in the portion of the enclosure surrounding the doorway and is fastened thereto using screws or rivets, or a conductive adhesive. The closed door contacts the fingers, thus providing a shield surrounding the door to a housing. See also U.S. Pat. No. 3,504,095 issued Mar. 31, 1970, to Robertson et al. Another configuration for shielding a door to a housing is disclosed in U.S. Pat. No. 4,864,076 issued Sep. 5, 1989, to Stickney, which illustrates a thin walled metal strip covering a portion of a sealing gasket.
U.S. Pat. No. 4,754,101 issued Jun. 28, 1988, to Stickney et al. relates to a shielding housing for enclosing a specific device or set of devices mounted on a conventional printed circuit board. The shield is a box formed from strips of metal having a plurality of downwardly-extending fingers and mounting prongs. The strips are fastened to a printed circuit board by soldering the mounting prongs into vias located on the circuit board, with the result that the housing covers the components the designer has assumed may require shielding.
Although the aforementioned solutions can be effective for specific applications requiring EMI shielding, each addresses a specific application and fails to address the problems raised by the unpredictability of EMI for a broad-based array of applications. An experienced designer can take measures to shield those devices which are known either to be EMI emitters or to be susceptible to emissions. Nonetheless, electromagnetic field interactions are sufficiently complex that unexpected stray currents or electrostatic fields may be and often are present. Generally such unpredicted (and unpredictable) EMI requires additional shielding components to be added manually to the system after it has been designed and tested. For example, in cellular radiotelephones, shielding elements such as copper tape and hand soldered grounding strips are commonly used--even on circuit boards manufactured entirely using modern automated equipment.
Manual EMI-addressing operations may contribute significantly to the cost and quality of manufacture. In some cases, a custom shield can be integrated into the manufacturing process, but this takes time and does not often provide a permanent solution. Varying manufacturing tolerances over different lots will often change the nature and source of EMI throughout the product lifetime, which then requires readjustment of the locations of the later-added shielding elements. As a result, a more flexible approach to solving EMI problems that can provide shielding for a broad range of applications and that can be integrated into existing automated manufacturing processes with little or no redesign of circuit boards is needed.